Spurious dashboard message: Xsara Picasso 90Hdi late 2004

[rmunns]

We are getting more-and-more frequently a message up on the display. Message along the lines of 'STOP - engine overheating'. This is when you start the car, no matter whether engine is warm or cold. At the same time the temp display is in the red.
Display clears itself after a few seconds and the temp. display goes to what it should be. Car has done less than 50k.
Any advice, please? I don't really want to start poking about at random in case this escalates into something more troublesome.

[Lighty]

This is usually the water temp transmitter failing, it is in the plastic thermostat housing , where the top hose goes to on the engine. The terminal colour is usually green, and the switch is held in with a horseshoe clip.

[rmunns]

Thanks for reply, Lighty (BTW, sat in a Twizy yesterday).

On our car I can't see any wire inserted into Thermostat hsg. Can't find way of uploading pic that I took.

[Lighty]